The processing of floor coatings comprising reactive methyl acrylate or methacrylate resins is normally associated with a strong offensive odor and significant air born toxicity. Furthermore, the low flash point and high volatility of methyl (meth)acrylate monomer add costs when it is handled during manufacturing, packaging, transportation and application. The term “(meth)acrylate” is used throughout this application to mean “acrylate or methacrylate”.
Polymerization of a mixture comprising methyl (meth)acrylate monomers, and a non-reactive (meth)acrylate bead polymer with an amine synergist, a peroxide initiator such as benzoyl peroxide, and wax as an oxygen barrier to form a (meth)acrylate floor coating resin has been broadly practiced in flooring and construction industry. The advantage of this technology is that it can be completely cured in one hour, whereas other coatings typically require much longer times, often six hours or more. Cured acrylate films exhibit excellent chemical and abrasion resistance, but many such coatings have issues with odor, volatility, toxicity and flammability.
Low-odor (meth)acrylate coating systems are known. Controlling the odor, in many instances, also somewhat lowers the toxicity as many of the volatiles responsible for the odor also contribute to the toxicity. Many of the low odor coatings however are prepared using only slightly higher MW monomers because incorporation of larger compounds, such as higher oligomers and polymers, often complicate application of the coating, especially in low solvent or solvent free coatings. Even for these low odor systems it is still difficult to achieve 100% cure while maintaining the good balance between the work time and curing time, and health risks often remain a concern. The need for a low-odor, environmentally friendly, non-flammable, cold-curing (meth)acrylate reactive resin composition for a floor coating with low health risks during application remains.
Historically, coating materials comprising thermoplastic and thermosetting polymers were applied from solvents that were subsequently removed by evaporation. However, changing safety, health and environment legislation have restricted solvent emissions and solvent-free coating materials and lacquers have become much more common.
UV curable coatings offer fast curing compared to other methods, such as moisture curing or thermal curing. Typical UV lacquers or varnishes are applied at room temperature and often comprise epoxy acrylate, polyester acrylate or urethane acrylate oligomers combined with acrylate functional monomers in the presence of photoinitiators. Under suitable UV wavelengths, the photoinitiators produce free radicals, which polymerize the acrylate functional groups to produce a cross-linked network. The relatively high concentrations of low viscosity reactive monomers often employed to reduce the viscosity of the coating composition in order to get good flow and leveling at room temperature can penetrate too deeply into porous substrates such as wood to be efficiently cured by UV radiation. Furthermore, UV curable films typically only cure well at low film thicknesses, e.g., 2 to 3 mils or less, with thicker films exhibiting surface flaws and other failures. Uncured monomer in the pores of substrates can lead to safety, health and environmental problems, e.g., when the materials are cut or sanded.
The use of a solvent-free reactive hot melt layer based on polyurethanes and hardened by atmospheric humidity has been suggested. While this method has the advantage that desired layer thicknesses can be applied in a single operation, full curing can take least several days to occur and the coated part cannot be processed or packaged rapidly.
Hot melt compositions that can be cured through both radiation and moisture or by applying a UV curable lacquer on top of the moisture curable hot melt are known. Generally the coating is partially cured by UV radiation, typically the top layer of portion of the coating, but full cure of the full coating may still take several days. Heating the hot melt composition prior to application also tends to limit thermal stability and can result in moisture from the atmosphere penetrating the composition and reacting with reactive functional groups, e.g., isocyanate groups. This leads to an increase in molecular weight while on the roller causing application problems, e.g., stringing or filament formation that can produce fouling of substrates/application equipment.
US 2009/0082485 discloses radiation curable hot melt coating compositions and articles comprising them. Typical compositions comprise a polymer component with number average molecular weight (Mn)>2,000 g per mole comprising polyurethane and/or poly(meth)acrylate segments and having at least one functional group polymerisable under UV radiation; an oligomeric component with Mn=500 to 5,000 g per mole comprising at least two functional groups polymerisable under UV radiation; at least one photoinitiator; and optionally either in place of or in addition to the oligomeric component, a monomer component with Mn=100 to 1,000 g per mole with at least one functional group polymerisable under UV radiation.
U.S. Pat. No. 7,049,355 discloses a low odor thermal curable floor coating formulation comprising low volatile methacrylate monomers, a non-reactive bead polymer, a paraffin and/or wax, and a redox system, containing an accelerator and a peroxide catalyst or initiator in an amount adequate for cold-curing the methacrylate monomers.
Co-pending U.S. patent application Ser. No. 14/584,344, discloses a curable, low odor, liquid coating composition, which is readily applied at room temperature without heating, useful, for example, as a coating for flooring applications, comprising a) a reactive acrylic based polymer comprising pendant acrylate or methacrylate groups; b) reactive unsaturated monomers, e.g., acrylic or methacrylic monomers; c) a thermal or UV activated radical initiator; d) an inert and non-polar wax or viscous oil, e.g., and e) optionally a tertiary amine accelerator used with a thermally activated radical initiator.
Safe, fast curing coating compositions with less toxicity, decreased flammability and better physical properties are still needed. The present invention provides coating compositions that are capable of curing via multiple pathways, e.g., free radical polymerization of (meth)acrylate moieties and urethane step chain polymerization of isocyanates, which compositions when cured exhibit improved strength and mechanical properties. The coating compositions of the present invention are readily applied, cure quickly and safely and are ideally suited for floor coatings and other industrial applications.